This invention is a method to improve certain direct plating processes by making volumetric additions. In the manufacture of printed circuit boards, holes are often drilled or punched through dielectric sheets clad on both sides with copper foil. The walls of the holes through the dielectric are then rendered electrically conductive by application of a thin conductive film. A much thicker deposit of electroplated metal, usually copper, is then applied to the walls of the through holes. The electroplated deposits form a physically strong, shock resistant electrical connection between the copper on one surface of the dielectric and the copper on the opposite surface of the dielectric. Numerous types of films have been used to render the walls of the through holes conductive including: electroless copper deposits, electroless nickel deposits, carbon black suspensions, graphite suspensions, and precious metal colloidal suspensions. This invention deals with colloidal suspensions of precious metals, especially palladium, and most especially with palladium/tin colloids, used to render the hole walls conductive in preparation for subsequent electroplating. This invention deals particularly with palladium/tin colloids disclosed in U.S. Pat. No. 4,933,000 issued to Okabayashi and the method of using such colloids for making nonconductive surfaces conductive as disclosed in U.S. Pat. No. 5,071,517 issued to Okabayashi.
Colloidal suspensions used to make nonconductive surfaces conductive are made of a tin stabilized precious metal colloid dispersed in an acidic, aqueous solution of a salt, usually sodium chloride. The precious metal colloid is called the catalyst. The acidic, aqueous salt solution is called the carrier. When mixed together the precious metal colloid and the carrier are called the catalyst bath. The salt concentration in the carrier solution is often near the maximum amount that is soluble in aqueous solutions at normal operating temperatures. The most commonly used catalyst in printed wiring board manufacture is tin stabilized palladium colloid. The catalyst disclosed in the above referenced patents also contains an organic stabilizing ingredient, usually an aldehyde, such as vanillin, dissolved in the aqueous salt solution.
One of the problems with this process is salt crystallization. In the normal operation of catalyst baths, the temperature of the bath is maintained at approximately 105.degree. F. Water evaporates from the bath. As water evaporates, salt crystals form in the bath. The crystals accumulate on the walls and bottom of the tank in hard agglomerations. The agglomerations coat heat transfer surfaces reducing heating efficiency.
Under production conditions, volume losses from evaporation arc often replaced with carrier solution. The carrier solution contains a substantial quantity of salt. As more water evaporates, the salt from the additions of carrier also crystallize from the bath.
In dip applications, when the salt deposits collect on the tank walls, they reduce the clearance between baskets used to hold parts and the side walls of the tanks. This makes it difficult to insert baskets into the tank. When salt crystals accumulate on the bottom of the tank, they can keep the baskets from being fully immersed in the bath, thus preventing processing of some areas of the parts in the baskets.
In applications when the bath is pumped, sprayed, or flooded onto parts, the salt deposits often break loose from the surfaces where they have accumulated. Particles of crystallized salt clog screens, pumps, flood heads, and nozzles of the equipment. The salt crystals can also clog the through holes in the work pieces, resulting in incomplete coverage of the hole walls by the colloidal catalyst bath. This can result in defects in the finished workpieces, which make them unsuitable for use. This in turn results in rejected parts and excessive cost. Removing clogging salt crystals from screens, nozzles, flood heads and other parts of the pumping system often requires that the manufacturing process be halted while the particles are removed. This can result in undesirable downtime for the process of building the parts.
In the past, when it became necessary to remove the salt deposits from process tanks, the crystals were mechanically broken loose from the tank and/or heater surfaces and scooped out of the bath. Alternatively, the bath would be allowed to cool to crystallize as much of the excess salt as possible. The cooled bath would be pumped out of the process tank into a collection vessel. Water would be placed in the emptied process tank and stirred to dissolve the excess salt deposits. Often hot water would be used or the water in the tank would be heated to hasten dissolution of the salt. Both of these cleaning procedures required interruption of production, were labor intensive, and were dirty operations.
Up to the time of this invention, it was believed that additions of water adequate to avoid crystallization of salt in the bath, would cause deterioration of the tin/palladium colloid of the catalyst, presumably due to hydrolysis. Producers of colloidal catalyst warn against adding water to the catalyst bath. This is specifically stated in Operating Instructions for IN 504 Activator written by Solution Technology Systems, an assignee of Okabayashi. In fact, laboratory tests have shown that additions of water to catalyst baths can cause deterioration of the colloid and resulting poor performance. When large volumes of water are added rapidly to catalyst baths, the normally uniform, opaque, black colloidal suspension of tin stabilized precious metal becomes lighter in color and grainy and non-uniform in appearance. In severe cases, such water contaminated catalyst baths may separate on standing. The particles of precious metal in the destabilized colloid no longer remain in suspension. Instead, they fall to the bottom of the container. Such separated baths are ineffective. Very small additions of water added very slowly with vigorous mixing have less adverse effect on the catalyst. But the small size and slow rate of additions as well as the excessive mixing required to avoid damage to the stability and performance of the catalyst make adding water directly to the catalyst bath unsatisfactory under production conditions.
The performance of the catalyst bath can be measured indirectly by testing the side-to-side resistance of catalyst activated copper clad dielectric workpieces with through holes. This is done using an ohmmeter and placing one contact on each planar side of the workpiece. Before treatment of the workpiece, the meter shows an open that is over one mega-ohm of resistance. After treating the workpiece with the colloidal catalyst bath, the resistance will be reduced. Prior art teaching the direct plating processes, using such tin/palladium catalysts as discussed herein, also teaches that on similar workpieces, the lower the side-to-side resistance of the catalyst activated workpieces, the more effective is the activation of the walls of the through holes. Although measuring the side to side resistance provides numerical values for effectiveness of activation, it is an indirect measurement.
Another method to test performance of a catalyst bath is to electroplate the copper clad workpiece after processing through the entire activation process. The walls of the plated through holes are then visually inspected under magnification, usually with a 10 to 15 power magnifier. The hole walls will be fully covered by the electroplated deposit if the catalyst is performing properly. The relative smoothness of the plated deposit in the hole is used to estimate the quality of the activation by the catalyst. The smoother and more uniform the electroplated deposit, the better the activation is considered to be. This method provides only a qualitative evaluation of the performance of the catalyst, but it directly determines the actually performance of the catalyst.
Catalyst baths that receive additions of water so that the bath appearance becomes non-uniform and light colored, yield side to side resistance readings much greater than similar baths not receiving the detrimental additions of water. The additions of water reduce the effectiveness of the bath. Additionally, the hole walls activated with such water contaminated baths often exhibit incomplete coverage after electroplating. For that reason it is customary to take care to minimize introduction of water into the catalyst bath. Before water wetted parts are placed in catalyst baths, they are typically first dipped into or otherwise contacted with a solution containing all the components of the catalyst bath except the precious metal colloid. This solution is known as a predip bath. The function of the predip bath is to displace and/or absorb water from the parts so as to reduce the amount of water introduced into the catalyst bath.